Many medicament delivery devices have an automatic dose delivery function that often consists of drive springs that are capable of acting on movable stoppers arranged inside medicament containers for expelling a dose of medicament through a medicament delivery member such as e.g. an injection needle.
The automatic delivery mechanism in a medicament delivery device often includes a compressed helical drive spring acting on a plunger rod to urge it in the proximal direction of the device upon activation. A compressed compression spring provides a force that is large enough to overcome the so called break loose force of the stopper, i.e. release the stopper from the inner wall of the medicament container, and the spring at its full extension, which usually is not the full decompression, provides a force that is large enough to complete the dose delivery but also can cause a breakage of the glass medicament container.
In order to handle this, some devices utilize so called constant force springs which are coil springs of strip material which have been shaped and pre-tensioned into tightly wound rolls. The spring is used in a linear movement, e.g. moving a plunger rod in the proximal direction, and produces almost constant force throughout its deflection. Document U.S. Pat. No. 5,478,316 discloses a medicament delivery device utilizing such a spring for driving a plunger rod during dose delivery. Further, it is sometimes desirable to have different force levels during different operations such as e.g. penetration and subsequent injection, but constant force during each operation.
Document EP 0 953 122 discloses such a spring. Although this type of springs is better in many aspects than the compression springs, they still have drawbacks. One such drawback is that the load on a plunger rod is non-symmetrical due to that the spring runs on the side of and parallel with the plunger rod, whereby a bending force is exerted on the plunger rod. The bending force could cause a breaking of the plunger rod or a deflection of the plunger rod such that it comes in contact with other components of the device, leading to increased friction or even jamming of the plunger rod.
A solution to these problems is known from document US 200610186143A1 wherein the load on the plunger rod is symmetrical. However, there is still rum for improvements wherein the device can be constructed with a few number of components where each component can achieve different functions.